Relatively large antennas such as dipoles are known. Unfortunately if used on a hand held portable radio such a dipole is generally relatively large with respect to the size of the portable radio. The large size of such a dipole antenna makes it undesirable for portable radio applications. One solution to the above antenna size problem is to use a monopole antenna instead. It is well established in the field of antennas that a monopole mounted perpendicularly to a conducting surface provides an antenna having good radiation characteristics, desirable drive point impedance, and relatively simple construction. Functionally, such a monopole structure may be viewed as an asymmetric dipole in which the monopole radiating element is one element and a radio case is the other element, or the counterpoise.
A further reduction of the physical size of the antenna has generally been achieved by employing a helically wound radiator, instead of a straight wire radiator, as the monopole radiating element. Thus, the helical element occupies significantly less physical length than the corresponding straight wire radiator, but desirably exhibits the same effective electrical length.
Physical size reduction, however, reduces the operating or radiation bandwidth of the antenna because of changes in the input impedance over frequency. Furthermore, wire antennas, being good conductors, possess low resistance and hence a high Q and a low radiation bandwidth result.
One solution to the narrow bandwidth problem has been disclosed in U.S. Pat. No. 4,772,895, assigned to the assignee of the present invention, which is hereby incorporated by reference. There, two helical elements are coupled together in a fashion which results in a dramatic increase in antenna bandwidth in comparison to prior helical antennas.
Aside from size consideration, there is also a manufacturability consideration. Maintaining, controlling, or fabricating the proper pitch angle or spacing in a helical element for consistency is challenging. There is, therefore, a need for a monopole radiating structure that also provides ease of manufacturability.